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Claim of reversed human evolution provokes skepticism, interest
Feb. 27,
2006
Special to World Science
Scientists’ reactions have ranged from deep skepticism to interest in a report of a mutation that makes people walk on all fours, cited in a Turkish study as a possible instance of “backward evolution.”
The bizarre case, reported last week in World
Science, has also drawn attention from several scientists in Europe, some of whom are
collaborating on a BBC documentary on it.
Three researchers with the University of Cambridge, U.K., and the London School of Economics wrote
recently that the case could represent a “rediscovery” of a walking style much like that of
human ancestors.
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An affected patient tries to stand.
Normally, they walk on all fours. (More pictures and video available here.)
(Courtesy Uner Tan)
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This might help resolve a debate over how our forebears walked, they
added.
The mutation is documented only in five members of a Turkish
family, whom researchers also describe as mentally and verbally
underdeveloped.
Uner Tan of Cukurova University Medical School in Adana, Turkey, has
also studied them. Going further than the British researchers, he has
claimed they might represent “backward evolution,” citing what he
called their ape-like gait and primitive
language.
He published his theory in the March issue of the
International Journal of Neuroscience.
Reverse evolution isn’t a new concept; many biologists accept it as a real phenomenon.
But it is controversial and somewhat ill-defined.
Several researchers disputed the idea that the syndrome might represent reversion or backward evolution.
The claim is “untestable,” wrote Henrique Teotónio, an evolutionary geneticist at the Gulbenkian Science Institute in Oneiras, Portugal, in an email.
It’s “impossible to judge it,” because scientists don’t know which
genes underlay human evolution, added Teotónio,
who has studied reverse evolution in flies.
He and others also said a key implication of Tan’s theory is doubtful: that just
one or a few genes could produce a complex trait such as upright walking—al though
new studies have surprised scientists with how much one gene can do,
such as orchestrating elaborate courtship rituals in flies.
Tan conceded his theory is debatable.
But he pointed out in his defense that new studies have mapped the defect to a region of the genome
called 17p, where some of the greatest differences between humans and
chimps lie. Researchers suspect such areas may be important in
human evolution.
German scientists published research linking the defect to 17p in the Dec. 21 issue of the Journal of Medical
Genetics. They wrote that this might help explain “the evolution of this unique hominid trait,”
upright walking.
The British team argued in a report dated Oct. 3, part of a discussion
paper series the Centre publishes, that it’s possible—but debatable—that “we are indeed seeing the ‘rediscovery’ of something very like the quadrupedal [four-limbed] gait used by our ancestors.”
Carriers of the mutation, they wrote,
all children of one couple, “as adults have continued to walk—highly
effectively—on hands and feet.” This gait, they added, seems to be a
development of a type of crawl that some children take up as a
transitional stage before upright walking.
Affected people walk palms down, unlike great apes, which walk on their
knuckles, wrote the British group, which includes the London School’s
Nicholas Humphrey.
“The local villagers laugh at and tease” the victims, they added. “Because of this, the females tend to stay close to the house, but the male sometimes wanders for several kilometres. He helps raise money for his family by collecting cans and bottles, which he carries home in a pouch made from his shirt, held by his teeth.
He is remarkably agile.”
The British group portrayed the victims’ language abilities more
generously than did Tan, who wrote that they speak a “primitive”
language of a few hundred words.
“They can all speak and understand Kurdish well enough to communicate within their own family, and three of them also speak some
Turkish,” Humphrey’s group wrote. “But their articulation is poor, and it seems they have a restricted vocabulary” and
problems with word arrangement. However, “They interacted with us as visitors in a friendly and courteous way.”
Although they can walk on two legs briefly, the team noted, they prefer
to go on all fours.
Based on brain scans, the scientists ascribed the condition to a type of disorder called an
ataxia, tied to underdevelopment of the cerebellum, a brain structure. Yet
this can’t be the whole explanation, they added; it could be “a combination of unusual factors—genetic, physiological, psychological and
social.”
The German team noted that affected people also have abnormally small heads, a condition called microcephaly,
which Charles Darwin cited as a possible evolutionary “regression” in his
On the Origin of Species.
If affected people do walk as our ancestors did, this might shed light
on a longstanding debate over how they did, Humphrey’s group argued. Various theories
claim humans evolved either from knuckle-walkers, who walked on their knuckles;
from tree climbers; or from “wrist-walkers” who moved on the ground with palms
down, the team wrote.
A fossil analysis published in the March 23, 2000 issue of the research journal
Nature pronounced results in favor of knuckle-walking. “But the discovery of these human wrist-walkers changes the situation,” Humphrey’s team wrote.
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